Method and apparatus for split radiography

ABSTRACT

In an equalization radiography device an X-ray fan beam (3,4) is used to scan a body (7) under examination. The X-ray fan beam comprises a number of adjacent sectors. A modulator device (5) comprising a number of controllable beam sector modulators is present for controlling per sector the quantity of X-ray radiation transmitted. A detector (11) placed behind the body generates per sector a measurement signal representing a desired position of the beam sector modulator for that sector. During operation the instantaneous position of each beam sector modulator is continuously detected (20,21). Signals representing those positions are generated and compared with the respective measurement signals. Control signals are derived from the comparisons to control the positions of the respective beam sector modulators.

The invention relates to a method for slit radiography, in which, withthe aid of an X-ray source and a slit-type diaphragm placed in front ofthe X-ray source, a fan-type X-ray beam is formed, with which beam abody under examination is scanned at least partially in a directiontransverse to the longitudinal direction of the slit of the slit-typediaphragm in order to form an X-ray shadow image on an X-ray detectorplaced behind the body, which fan-type X-ray beam is formed by amultiplicity of sectors situated adjacently to one another, thetransmitted X-ray radiation being influenced instantaneously during thescanning movement per sector of the fan-type beam, while in operation,by means of controllable beam sector modulators interacting with theslit diaphragm, the quantity of radiation transmitted through the bodybeing measured with the aid of detection means instantaneously persector of the X-ray beam during the scanning movement and themeasurement result being used to control the beam sector modulators. Theinvention furthermore relates to an apparatus for applying the method.

Such a method and such an apparatus are known from the Dutch PatentApplication 84.00845. According to the technique known from the DutchPatent Application 84.00845, to regulate the quantity of x-ray radiationtransmitted through the slit diaphragm at any instant in time, use ismade of attenuating devices which are placed near or in the slit of theslit diaphragm and act as beam sector modulators, which can eachinfluence a sector of the fan-type X-ray beam and which, depending onthe attenuation occurring in the associated sector and caused by thebody under examination, are controlled in a manner such that theattenuating devices extend to a greater or lesser degree into the X-raybeam. If the attenuation due to the transradiated body is large in aparticular sector at a particular instant, the attenuation device.associated with the sector is moved completely or largely out of theX-ray beam. On the other hand, if the attenuation due to the body is lowin a particular sector at a particular instant, the associatedattenuation device is brought further into the X-ray beam.

The advantage of this technique is that equalized radiographs canthereby be obtained, that is to say, radiographs which have a goodcontrast both in the light parts and in the dark parts. Therefore if,for example, a radiograph is made in this manner of the upper part ofthe body of a patient, the radiologist is able to find, in one and thesame radiograph, adequate information for both the chest and theabdominal cavity of the patient, whereas two different radiographs werehitherto necessary to obtain the same information.

One problem in the known method is that the beam sector modulators mayexhibit hysteresis phenomena. These phenomena occur, in particular, ifpiezoelectric tongues are used as (carriers of) absorption devices, butalso, for example, in the case of beam sector modulators which comprisespring devices or are linked thereto.

As a consequence of such hysteresis phenomena, for example, the positionof a beam sector modulator with respect to the beam sector to beinfluenced may deviate from the position which corresponds to thesignals provided by the detection means. Undesirable artefacts mayconsequently be produced in the final X-ray shadow image.

The object of the invention is to eliminate, or at least to reduce, theproblem outlined.

For this purpose, according to the invention, a method of the typedescribed is characterized in that, during operation, the instantaneousposition of each beam sector modulator is continuously detected, in thatan electrical signal representing the instantaneous position isgenerated for each beam sector modulator, in that the electrical signalrepresenting the instantaneous position is compared with the measurementresult provided by the detection means and associated with therespective beam sector, and in that a control signal for the respectivebeam sector modulator is formed from the measurement result and thesignal representing the instantaneous position.

A slit radiography apparatus comprising an X-ray source which is able toscan, at least partially, via a slit or a slit diaphragm a body underexamination with a fan-type beam in a direction transverse to thelongitudinal direction of the slit in order to form an X-ray shadowimage on an X-ray detector, beam sector modulators interacting with theslit diaphragm which, during operation, are able to influence thefan-type beam instantaneously per sector during the scanning movement inorder to be able to regulate the X-ray radiation incident in each sectoron the body under examination, and detection means which are designed todetect, during a scanning movement of the X-ray beam, the quantity ofX-ray radiation transmitted through the body instantaneously per sectorand to convert it into corresponding signals, is characterized,according to the invention, by means which, during operation, are ableto detect the instantaneous position of each beam sector modulator andare able to provide electrical signals corresponding to the detectedpositions, and by means which are able to form control signals for thebeam sector modulators from the said electrical signals and the signalsprovided by the detection means for the quantity of radiationtransmitted through a body.

The invention will be described in more detail below with reference tothe accompanying drawing.

FIG. 1 shows diagrammatically an example of a known slit radiographyapparatus;

FIG. 2 shows diagrammatically an exemplary embodiment of an apparatusaccording to the invention;

FIG. 3 shows diagrammatically a variant of a part of FIG. 2; and

FIG. 4 further shows diagrammatically a variant of a part of FIG. 3.

FIG. 5 shows diagrammatically a variant of FIG. 3.

FIG. 1 shows diagrammatically an example of a known slit radiographyapparatus. The slit radiography apparatus shown comprises an X-raysource 1 having an X-ray focus f. Placed in front of the X-ray source isa slit diaphragm 2 having a slit 3 which, during operation, transmits anessentially flat, fan-type X-ray beam 4. A beam sector modulation system5 is furthermore present which is able to influence the fan-type X-raybeam per sector thereof. The beam sector modulation system is controlledby means of regulating signals supplied via a conductor 6.

During operation, the X-ray beam 4 transradiates a body 7 underexamination. Placed behind the body 7 is an X-ray detector 8 forrecording the X-ray shadow image. The X-ray detector 8 may, for example,be a large format cassette as shown in FIG. 1, but it may also be, forexample, a moving elongated X-ray image intensifier.

In order to form an image, on the x-ray detector, of the entire body 7or at least a part thereof which is under examination such as thethorax, the fan-type X-ray beam executes, during operation, a scanningmovement such as is indicated diagrammatically by an arrow 9. For thispurpose, the X-ray source together with the slit diaphragm 2 and thesystem 5 may be arranged pivotably with respect to the X-ray focus f asindicated by an arrow 10. However, it is also possible to scan a bodyunder examination with a flat X-ray beam in a different manner, forexample by causing the X-ray source to execute a linear movementtogether with or without the slit diaphragm.

Placed between the body- 7 and the X-ray detector 8 are detection means11 which are designed to detect the quantity of radiation transmittedthrough the body instantaneously per sector of the fan-type beam 4 andto convert it into, corresponding electrical signals which are fed viaan electrical connection 12 to a regulating system 13 which formsregulating signals for the modulation system 5 from the input signals.The detection means 11 may comprise, for example, a one-dimensionalstationary dosimeter which extends essentially parallel to the X-raydetector or the plane in which the latter executes a scanning movement.The dosimeter has dimensions such that it covers the entire width of theregion scanned by the flat X-ray beam during operation and is moved,during operation, synchronously up and down with the X-ray beam as shownby the arrows 14. The dosimeter has been described above as aone-dimensional dosimeter. This term is not mathematically correct, butthe thickness of the dosimeter is relatively low when viewed in thedirection of the X-ray radiation.

Suitable dosimeters may comprise an ionization chamber divided intosections and are, for example, described in the Applicant's Dutch PatentApplications 85.03152 and 85.03153. It is pointed out that the detectionmeans may also be placed behind the X-ray screen 8, for example in themanner described in the Dutch Patent Application 84.00845. Furthermore,a two-dimensional dosimeter such as that described, for example, in theApplicant's earlier Dutch Patent Application 87.01122 may also be used.

As described in the Dutch Patent Application 84.00845, the beam sectormodulation system may comprise a multiplicity of tongues of, forexample, piezoelectric material placed next to one another and havingone end mounted on a carrier, the other, free end of which can bebrought to a greater or lesser degree into the X-ray beam under theinfluence of the regulating signals. The free ends of the tongues mayoptionally furthermore be provided with separate absorption devices of amaterial which absorbs X-ray radiation. Such a tongue-type modulator isshown diagrammatically at 15 in FIG. 1 by way of example, but within thescope of the invention, other types of beam sector modulators can alsobe used.

As already noted, hysteresis phenomena which have the result that thebeam sector modulators assume a position with respect to the X-ray beamother than that corresponding to the regulating signals supplied, mayoccur in practice in controlling the beam sector modulators.

These hysteresis phenomena may be the result of a mechanical hysteresissuch as occurs, for example, in the case of springs or of anelectromechanical hysteresis such as occurs in the case of piezoelectricdevices or of magnetic hysteresis such as occurs in the case of(electro)magnets.

The influence of the hysteresis phenomena may be eliminated or at leastreduced, according to the invention, by using one or more additionaldetectors which provide signals which precisely correspond to theinstantaneous positions of the beam sector modulators.

FIG. 2 shows diagrammatically a first embodiment of an apparatusaccording to the invention. In FIG. 2, the same reference numerals havebeen used for corresponding elements as in FIG. 1.

Placed between the X-ray source 1 and the beam sector modulation system5 is a first additional radiation detector 20 which is able to detectthe quantity of radiation provided per sector of the X-ray beam and isable to provide electrical signals corresponding thereto. A suitableradiation detector is, for example, the dosimeter described in theApplicant's Dutch Patent Application 85.03153. The radiation detector 20is placed, in the example shown, between the X-ray source 1 and the slitdiaphragm 2. The operating region of the detector should then correspondto that portion of the X-ray beam which can actually be transmittedthrough the slit 3 of the slit diaphragm. That can be achievedelectronically by processing the signal on the line 25 but screeningmeans may also be used for this purpose. The radiation detector 20 mayalso be placed between the diaphragm and the beam sector modulationsystem.

It is possible to provide the beam sector modulation system between theslit diaphragm and the X-ray source. In that case, the radiationdetector 20 should be situated between the X-ray source and the beamsector modulator.

A second radiation detector 21 is furthermore provided beyond the beamsector modulation system. The second radiation detector is able tomeasure the quantity of radiation instantaneously incident on the bodyunder examination per sector of the fan-type X-ray beam 4 and is able toprovide corresponding electrical signals.

Therefore, the difference in, or the ratio of, the output signals of thefirst and second radiation detector is a measure of the actual positionof each beam sector modulator for each beam sector.

As a result of then comparing this actual position with the desiredposition, control signals can be obtained with which the beam sectormodulators can be precisely controlled. By starting from the actualposition of the beam sector modulators, automatic compensation can beprovided for hysteresis effects.

Electrical signals which represent the desired position of the beamsector modulators are provided in a known manner by the detection means11 which are situated behind the body under examination. The signalsoriginating from the detection means are applied, possibly aftercomparison with a first reference signal in a differential amplifier 22,as a reference signal S₁ to a first input of a differential amplifier 23which receives, at the other input, a signal S₂ representing the actualposition of the beam sector modulator of the respective sector.

The signal S₂ is the output signal of a device 24 which receives theoutput signals of the first and second radiation detector via conductors25 and 26 and is able to compare said signals sector-wise with oneanother for providing, per sector, a signal S₂ which represents theactual position of the beam sector modulator associated with therespective sector. The device 24 may be, for example, a differentialamplifier or a divider.

Finally, the output signals S₃ of the differential amplifier 23 are usedas control signals for the beam sector modulators and are fed via aconductor 27 to the respective beam sector modulators or to the controldevices therefor.

The radiation detectors 20 and 21 may move concomitantly with thescanning movement of the X-ray source. As an alternative, the radiationdetectors 20 and 21 may be constructed as two-dimensional detectors asalready specified above for the detector 11.

It is also possible to construct, for example, the first radiationdetector 20 as a concomitantly moving one-dimensional detector and thesecond detector 21 as a two-dimensional detector as described, forexample, in the Applicant's earlier Dutch Patent Application 87.01122.

This and similar modifications are obvious to the person skilled in theart and are considered to fall within the scope of the invention.

According to an alternative elaboration of the inventive idea,instantaneous actual positions of the beam sector modulators may also bedetected in a different manner. Although use is preferably made ofcontactless position-determining methods, it is possible in principle tocouple each beam sector modulator mechanically to, for example, theslider of an adjustable resistor or the movable plate of an adjustablecapacitor. Use may also be made of diverse known types of displacementmeters such as, for example, coaxial capacitive displacement meters witha central electrode which is able to move inside an assembly ofcylindrical electrodes in accordance with the movement of a feeler arm.An inductive method of measurement in which each beam sector modulatoris coupled to a movable coil core may also be used.

It is also possible to use each beam sector modulator itself as theelectrode of a capacitor, or to provide a capacitor electrode in orderto determine the instantaneous position of each beam sector modulator ina capacitive manner with the aid of a suitable counterelectrode and asuitable measurement voltage.

FIG. 3 indicates diagrammatically, by way of example, a method in whicha tongue-type beam sector modulator 30 forms a movable capacitorelectrode which interacts with a fixed capacitor electrode 31. Asuitable measurement signal can be applied between the electrodes 30 and31, for example a high-frequency measuring voltage provided by ameasuring voltage source 32. The impedance of the circuit comprising thevariable capacitor 30, 31 depends on the position of the electrode 30.This can be measured in a known manner suitable for the purpose with theaid of a suitable detector 33. The detector 33 is designed in a mannersuch that it delivers a signal S₂ which is representative of theinstantaneous actual position of the beam sector modulator and which, asin the example of FIG. 2, is fed to a differential amplifier 23. In apractical embodiment, the electrode 31 may be a strip-type commonelectrode for all the beam sector modulators and the beam sectormodulators may be connected consecutively to the measuring signal source32 by means of an electronic or mechanical scanning system.

FIG. 4 shows diagrammatically, by way of example, a method ofdetermining the instantaneous position of a beam sector modulatoroptically. The tongue-type beam sector modulators 40 shown in theexample are illuminated by a light source 41. Situated at the other sideof the beam sector modulators is, for each beam sector modulator, alight detector 42, for example a photosensitive semiconductor device,which, depending on the size of the shadow region 43 due to the beamsector 25 modulator, delivers an electrical signal S₂ which is again fedto a differential amplifier 23 in the manner already described above.

It is true both of the embodiment of FIG. 3 and the embodiment of FIG. 4that the methods shown are suitable for differently formed beam sectormodulators with modifications which are obvious to the person skilled inthe art. The position of a fixed device coupled to the beam sectormodulator may also be determined instead of the position of a beamsector modulator itself.

FIG. 5 shows an inductive method of measurement in which each beamsector modulator is coupled to a movable core 36 of a coil 35.

After the above, this and similar modifications are obvious to theperson skilled in the art.

We claim:
 1. Method for slit radiography in which, with the aid of anX-ray source and a slit-type diaphragm placed in front of the X-raysource, a fan-type X-ray beam is formed, with which beam a body underexamination is scanned at least partially in a direction transverse tothe longitudinal direction of the slit of the slit-type diaphragm inorder to form an X-ray shadow image on an X-ray detector placed behindthe body, which fan-type X-ray beam is formed by a multiplicity ofsectors situated adjacently to one another, the transmitted X-rayradiation being influenced instantaneously during the scanning movementper sector of the fan-type beam, while in operation, by means ofcontrollable beam sector modulators interacting with the slit diaphragm,the quantity of radiation transmitted through the body being measuredwith the aid of detection means instantaneously per sector of the X-raybeam during the scanning movement and the measurement result being usedto control the beam sector modulators, characterized in that, duringoperation, the instantaneous position of each beam sector modulator iscontinuously detected, in that an electrical signal representing theinstantaneous position is generated for each beam sector modulator, inthat the electrical signal representing the instantaneous position iscompared with the measurement result provided by the detection means andassociated with the respective beam sector, and in that a control signalfor the respective beam sector modulator is formed from the measurementresult and the signal representing the instantaneous position.
 2. Methodaccording to claim 1, characterized in that the signal representing theinstantaneous position is obtained with the aid of a system of radiationdetectors having a first radiation detector which is placed between theX-ray source and the beam sector modulators the beam sector modulators.3. Method according to claim 1, characterized in that the signalrepresenting the instantaneous position is obtained with the aid of anelectrical measuring method in which the movement of each beam sectormodulator brings about an impedance change in a measuring circuit, whichimpedance change is detected and is converted into a signal representingthe instantaneous position.
 4. Method according to claim 1,characterized in that the signal representing the instantaneous positionis obtained with the aid of an optical measurement method in which themovement of each beam sector modulator brings about a change in thelight incident on an associated light detector and originating from alight source and in which each light detector provides an electricalsignal corresponding to the incident quantity of light
 5. Slitradiography apparatus comprising an X-ray source which is able to scan,at least partially, via a slit of a slit diaphragm a body underexamination with a fan-type beam in a direction transverse to thelongitudinal direction of the slit in order to form an X-ray shadowimage on an X-ray detector, beam sector modulators interacting with theslit diaphragm which, during operation, are able to influence thefan-type beam instantaneously per sector during the scanning movement inorder to be able to regulate the X-ray radiation incident in each sectoron the body under examination, and detection means which are designed todetect, during a scanning movement of the X-ray beam, the quantity ofX-ray radiation transmitted through the body instantaneously per sectorand to convert it into corresponding signals, which apparatus ischaracterized by means which, during operation, are able to detect theinstantaneous position of each beam sector modulator and are able toprovide electrical signals corresponding to the detected positions, andby means which are able to form control signals for the beam sectormodulators from the said electrical signals and the signals provided bythe detection means for the quantity of radiation transmitted through abody.
 6. Apparatus according to claim 5, characterized in that the meansfor detecting the instantaneous position of the beam sector modulatorscomprise a first radiation detector placed between the X-ray source andthe beam sector modulators and a second radiation detector placedbetween the beam sector modulators and the body under examination. 7.Apparatus according to claim 6, characterized in that at least oneradiation detector consists of an elongated ionization chamber which isdivided into sections corresponding to the beam sectors.
 8. Apparatusaccording to claim 5 or 6, characterized in that at least one radiationdetector consists of a two-dimensional ionization chamber which isdivided into sections corresponding to the beam sectors.
 9. Apparatusaccording to one of claims 6 to 8 inclusively, characterized by acomparison system which compares the signals provided by the first andsecond radiation detectors per beam sector with one another and providescorresponding output signals, which output signals are fed to a firstinput of a differential amplifier which receives at the other inputsignals which represent the quantity of radiation detected in thecorresponding sector by the detection means and transmitted by the bodyunder examination, which differential amplifier provides, at the output,control signals for the beam sector modulator associated with therespective sector.
 10. Apparatus according to claim 9, characterized inthat the comparison system comprises a divider.
 11. Apparatus accordingto claim 9, characterized in that the comparison system comprises adifferential amplifier.
 12. Apparatus according to claim 5,characterized in that the means for detecting the instantaneous positionof the beam sector modulators comprise a measuring circuit for each beamsector modulator, which measuring circuit is provided with a measuringsignal source and a variable electrical impedance component, theactuating device of the variable electrical impedance component beingmechanically coupled to the respective beam sector modulator in order totransmit the movement of the beam sector modulator.
 13. Apparatusaccording to claim 5, characterized in that the means for detecting theinstantaneous position of the beam sector modulators comprise ameasuring circuit for each beam sector modulator, which measuringcircuit is provided with a measuring signal source and a variableelectrical impedance component, the electrical impedance componentforming a reactive impedance which can be contactlessly changed. 14.Apparatus according to claim 13, characterized in that the reactiveimpedance comprises a coil with a core which can be moved relativelywith respect to the coil.
 15. Apparatus according to claim 13,characterized in that the reactive impedance comprises a capacitorhaving a movable electrode and a fixed counterelectrode.
 16. Apparatusaccording to claim 15, characterized in that the fixed counterelectrodeis a common counter electrode for at least a number of beam sectormodulators.
 17. Apparatus according to claim 15 or 16, characterized inthat each beam sector modulator itself embodies a movable electrode. 18.Apparatus according to claim 5, characterized in that the means fordetecting the instantaneous position of the beam sector modulatorscomprise illumination means for each beam sector modulator or alight-intercepting device coupled thereto and also light detection meanswhich are able to detect by means of the size of the shadow cast by eachbeam sector modulator and to convert the magnitude into an electricalsignal.